Hydraulically actuated free piston air compressor



Dec. 1, 1970 c. E. LILES 3,544,233

HYDRAULICALLY ACTUATED FREE PISTON AIR COMPRESSOR Filed Jan. .10, 1969 4 Sheets-Sheet 3 -J b00/ rmre/ AIIURA/EY/ 7 Filed Jan. 10. 1969 Dec. 1, 1970 c. E. LILES 3,544,238

HYDRAULICALLY ACTUATED FREE PISTON AIR COMPRESSOR 4 Sheets-Sheet 4 erren/fifime Perm/12 {flake United States Patent O 3 544,238 HYDRAULICALLY ACTUATED FREE PISTON AIR COMPRESSOR Clarence E. Liles, Los Angeles, Calif., assignor to White Motor Corporation, Cleveland, Ohio, a corporation of Ohio Filed Jan. 10, 1969, Ser. No. 790,269 Int. Cl. F04b 35/00, 1/00 U.S. C]. 417-397 28 Claims ABSTRACT OF THE DISCLOSURE In a hydraulically actuated free piston air compressor, a 4-way valve is shifted part way mechanically by the free piston and is shifted the rest of the way hydraulically. At a predetermined point intermediate in each opposite piston stroke, means is effective to increase the input area of the piston for a second stage of air compression.

BACKGROUND OF THE INVENTION While the application is applicable to air compressors for various specific purposes, it is particularly directed to the problem of providing an eflicient economical and relatively simple hydraulically actuated compressor to maintain a supply of pressurized air at an approximately 100-125 p.s.i. on a truck or tractor for actuating brakes and other pneumatically operated devices.

One object of the invention is to avoid the usual waste of power in the operation of such a compressor. A conventional compressor on a truck operates to compress air for only 5 to 15% of the truck operating time and during the rest of the truck operating time continues to consume power but unloads through a relief valve. The present invention provides for complete idleness of the compressor for 85 to 95% of the truck operating time.

Other objects of the invention relate to problems that are involved in the operation of a hydraulically actuated free piston air compressor.

An important one of these problems relates to the actuation of a 4-way valve for controlling the free piston. The spool of such a 4-way valve has a left limit position for driving the piston rightward, a right limit position to drive the piston leftward and a central null position at which it cuts oif hydraulic flow to both sides of the piston. The requirement is for some provision to shift the valve spool from one of the two limit positions to the other limit position as the piston reaches the end of a stroke thereby to reverse the piston. It is also required that some provision be made to keep the valve spool at whichever limit position it may be placed until the time comes to reverse the position of the valve spool.

Various solutions to this problem are found in the prior art. Some of the solutions are mechanical solutions wherein the piston mechanically shifts the valve spool or stores energy for the mechanical shift. Other of the solutions are hydraulical solutions wherein the spool is hydraulically unbalanced towards its two limit positions in turn. In general. the mechanical solutions are cumbersome and too often are unreliable in that it is possible for the valve spool to stall. Some of the hydraulic solutions are reliable but, in general, where the hydraulic solution is reliable, the valve shifting means is unduly complicated and the compressor is bulky rather than compact as required for use on an automotive truck.

One problem with which the invention is concerned is to reduce the power input that is necessary to operate a hydraulically operated free piston air compressor.

Another problem is to avoid the wide variations in the reservoir pressure that arise because a substantial drop in the pressure is required to start the compressor. A further problem is to decelerate the free piston as it approaches the end of each of its opposite strokes.

SUMMARY OF THE INVENTION With reference to the usual waste in power involved in the continued actuation of a truck air compressor when air compression is not needed is solved by employing a hydraulically actuated free pitosn compressor instead of a conventional compressor. The free piston compressor automatically stalls and thus becomes idle without consuming power whenever normal pressure is reached in the air reservoir. The substitution of a free piston compressor also avoids the usual drop in reservoir pressure that is required for restarting the compressor. The substituted free piston compressor starts operation in response to only a slight drop from normal reservoir pressure.

Reliable operation of a 4-way valve in a free piston pump is achieved by provision for the piston to start the shift of the valve spool as the piston reaches the end of a stroke and by the further provision of hydraulic means to complete the shift of the valve spool. The valve spool cannot stall because while it is moving from one limit position to another it is either under mechanical actuation by the piston or under hydraulic actuation, there being no intermediate position of the spool in its range of reciprocation in which it is not actuated either mechaincally or hydraulically.

A feature of the preferred practice of the invention is the provision of two modes of hydraulic actuation of the valve spool that work together to expedite the hydraulic shift of the valve spool to its opposite limit positions. In one of the hydraulic modes, a pair of 3-way valves operable as a 4-way valve controls the application of hydraulic fluid to the opposite ends of the valve spool in turn. In the other mode of hydraulic actuation the valve spool is shaped and dimensioned relative to the surrounding structure to provide differential areas for hydraulically biasing the valve spool towards its opposite limit positions in turn. At least one of the two modes of hydraulic actuation is not only effective for shifting the valve spool but is also eifective to hold the valve spool at a limit position throughout the major portion of each piston stroke.

With reference to making a hydraulically actuated free piston compressor compact and simple with a minimum number of working parts, a feature of the preferred practice of the invention is the concept of reciprocating the valve spool along the axis of the free piston with the free piston of annular configuration to surround the valve spool. Such an arrangement not only conserves space but also simplifies the problem of mechanical initiation of a valve spool shift by the piston, it merely being necessary to provide suitable portions of the valve spool in the path of reciprocation of the surrounding free piston.

In the preferred embodiment of the invention, compactness and simplicity is further achieved by uniting the two valve spools of the two 3-way valves to act as a 4-way valve that controls the application of hydraulic pressure to the opposite ends of the valve spool in turn. This concept is carried out by employing a composite valve spool wherein the central portion of the valve spool has the function of a 4-way valve spool and each of the two opposite end portions of the composite valve spool provide the functions of the two 3-way valves respectively.

With further reference to economical use of power for hydraulically actuating a free piston air compressor, an important feature of the invention is the provision of means to increase the ratio between the input area of the free piston and the output area of the free piston at a predetermined intermediate point in each of the opposite strokes of the piston. The result is that the air is compressed in two stages with substantial saving in power. In such an arrangement, there is always a possibility, however, that an exhaust valve of the air cylinder may function improperly to permit the air pressure in the :cylinder to rise high enough to cause the piston to stall during the first stage of compression before the piston reaches the predetermined point in its stroke. This particular problem is solved by providing a leakage path to initiatethe second stage of compression in advance of the predetermined point whenever there is any tendency for the piston to stall in the first stage.

The preferred embodiment of the invention is characterized by the free piston structure including two opposite axial sleeve extensions that slidingly embrace the valve spool and which have multiple functions for the purpose secondary input surfaces of the piston structure. A fourth 7 function of the sleeve extensions is to cooperate with the periphery of the valve spool to provide the previously mentioned leakage paths that permit the piston to enter its second stage whenever the piston tends to stall in the first stage. As will be explained, a fifth function of the sleeves is to cooperate with the valve spool to bias the spool by differential hydraulic pressure.

Operating the free piston through two stages of compression partially solves the problem of piston decelera- BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, which are to be regarded as merely illustrative FIG. 1 is a longitudinal sectional view of the principal working parts of the compressor wherein spaces under high hydraulic pressure are stippled;

FIGS. 2 and 3 are similar simplified longitudinal sec- Ytional views on a smaller scale showing the parts at different points in the operating cycle;

FIG. 4 is a diagram showing the mechanical and hydraulic forces that act on the valve spool throughout its range of reciprocation;

FIG. 5 is an enlarged fragmentary sectional view showing the valve spool at a rightward balanced position; a FIG. 6 is a similar view showing the valve member moved slightly leftward from the rightward balanced position;

FIG. 7 is simliar view of the valve spool at a leftward balanced position; v FIG. 8 is a similar view showing the valve spool moved slightly rightward from the leftward balanced position;

FIG. 9 is a graph showing the input horsepower at different points in the stroke of a single stage compressor; and

FIG. 10 is a similar graph showing the input horsepower at diiferent points in a two-stage stroke of the free piston.

4 DESCRIPTION OF THE- PREFERRED EMBODIMENT OF THE INVENTION In FIG. 1, the spaces under high hydraulic pressure are stippled, the spaces under low hydraulic pressure being left blank.

'Ihe compressor has fixed structure which forms an air cylinder 10 with two axial extensions 12 of reduced cross section, the air cylinder being provided with a flapper-type intake check valve 14 and a similar exhaust valve 15. A free piston, generally designated 16, of annular configuration has two opposite axial sleeve extensions 18 that slidingly extend into the opposite extensions 12 of the cylinder and the piston as well as the sleeve extensions slidingly embraces a valve spool that is generally designated 20. r

The free piston has a first outer radial portion 22 which lies outside the diameter of thesleeve extensions 18 and which provides the annular output area that acts against air in the cylinder 10. The free piston structure is further provided with a central inner circumferential land 24 that slidingly embraces a central portion 25 of the valve spool and is provided with an O-ring 26 for sealing engagement with the valve spool.

The inner circumferential land 24 constitutes a second portion of the piston which provides a primary input area for hydraulic actuation of the piston. The opposite end surfaces of the sleeve extensions 18 provide secondary input surfaces which are additionally acted on by the high pressure hydraulic fluid for the second stage of each piston stroke.

For the purpose of operatively connecting the piston to the valve spool 20 as the piston approaches each of its two opposite limit positions, the valve spool has two longitudinally spaced shoulders that face each other and the piston has two cooperating longitudinally spaced shoulders that face away from each other. The two shoulders of the valve spool are formed by two radial projections 27 and the cooperating shoulders of the piston are'formed by the outer ends of the sleeve extensions 18. Thus'as the piston approaches each of its two opposite limit positions, it encounters a radial projection 27 and shifts the valve spool out of one of its'limit positions part way towards its other limit position.

In the construction shown, the valve spool 20 is made in three longitudinal sections which are interconnected by the cross pins (not shown) and which telescope together with each telescoped joint sealed by an'O-ring 28. The left extension 12 of the cylinder 10 has an annular inlet port30 for high pressure'hydraulic fluid and the right cylinder extension has a similar annular inlet port 32, each of the two annular ports being in communication with a corresponding high pressure hydraulic passage 34. The left annular inner port 30 lies between an annular low pressure outlet port 35 and a second annular low p ressure outlet port 36 that is substantially wider than the first outlet port. In like manner the annular, inlet port 32in the right extension of the cylinder 10 lies between'an annular low pressure outlet port 38 and a wider annular low pressure outlet port 40. Each of the jpairsiof outlet ports 35, 36 and 38, 40 is in communication with a correspond.- ing low pressure hydraulic passage 42. In addition, each of the cylinder extensions .12 has a check valve 43 to permit inflow of low pressure hydraulic fluid whenever a vacuum tends to develop in the cylinder extension. The left end of the valve spool 20 is-"fo'rmed witha hydraulic chamber 44 in the form of an axial bore that slidingly telescopes over a corresponding floating plunger 45 so that hydraulic pressure in the. chamber exerts right ward pressure on the end of the valve spool and simultaneously urges the floating plunger leftward againstthe corresponding end wall 46 of the cylinder extension. In like manner, the right end of the valve spool has a hy draulic chamber in the form of an axial bore 48 and a corresponding floating plunger 50.

The valve spool 20 has a first set of passages and peripheral ports to cooperate with the high pressure inlet ports 30 and 32 and with the low pressure outlet ports 36 and 40 for 4-way valve action to control the opposite strokes of the piston. This set includes an axial passage 52 in the left end of the spool which is provided with radial ports 54 and 55, the radial ports 55 being in communication with the left side of the inner circumferential land 24, i.e. in communication with the primary input area on the left side of the piston. In addition, the axial passage 52 is provided with radial ports 56 for communication with the secondary input area on the end of the left sleeve extension 18 during the second stage of the rightward stroke of the piston.

The first set of passages and ports of the valve spool further includesan axial passage 60 in the right end of the spool which is provided with radial ports 62 and 64, the radial port 64 being in communication with the right side of the inner circumferential land 24, i.e. in communication with the primary input area on the right side of the piston. The axial passage 60 is further provided with radial ports 65 for communication with the secondary input area on the end of the right sleeve extension during the second stage of the leftward stroke of the piston.

In the position of the piston shown in FIG. 1 an inner circumferential land 58 of the right sleeve extension 18 covers the radial ports 65. If the piston is shifted rightward from the position shown in FIG. 1, the radial ports 65 communicate with the primary input area formed by the inner circumferential land 24 of the piston and if the piston is shifted leftward from the position shown in FIG. 1, the ports 65 communicate with the secondary input area formed by the end surface of the right sleeve extension 18. In like manner, an inner circumferential land 66 of the left sleeve extension 18 controls the radial ports 56.

The valve spool 20 has a second set of passages and peripheral ports to cooperate with the high pressure inlet ports 30 and 32 and with the low pressure outlet ports 35 and 38 for 3-way valve action to control hydraulic flow to the two opposite hydraulic chambers 44 and 48 at the opposite ends of the valve spool. This second set of passages and ports include the bore '44 in which the left floating plunger 45 is mounted together with a radial port 70 for communication with the inner end of the plunger and a peripheral port 72 for communication with the outer end of the plunger. In like manner, the second set of passages and ports of the valve spool 20 further includes the bore 48 in which the right floating plunger 50 is mounted together with a radial port 74 for communication with the inner end of the plunger and a peripheral port 75 for communication with the outer end of the plunger.

The mode of operation of the compressor may be understood by reference to FIGS. 2 and 3 wherein spaces that are for the moment under high hydraulic pressure are stippled, the passages that are under low hydraulic pressure being left blank. FIG. 3 shows the free piston 16 at an early point in its leftward stroke before the piston reaches its second stage.

With reference to the 4-way valve action for controlling the piston, FIG..2 shows the radial ports 62 at the right end of the valve spool in communication with the high pressure inlet port 32 to transmit high hydraulic pressure to the primary input area on the right side of the piston, i.e. the area of the right side of the inner circumferential land 24 of the piston. It is to be noted that at this time the land 58 of the right sleeve extension 1 8 of the piston cuts oflf flow of the high pressure hydraulic fluid to the outer end of the rightward sleeve extension which provides the secondary input area for the second stage of the leftward stroke. At the left end of the valve spool 6 the radial port 54 communicates with the low pressure outlet port 36 for the release of hydraulic fluid that is displaced by the leftward stroke of the piston.

When the leftwardly moving piston reaches the point shown in FIG. 3, the land 58 of the right sleeve extension 18 partially uncovers radial ports to start the second stage by the application of high hydraulic pressure to the outer end of the right sleeve extension.

When the piston is in its first stage at its leftward stroke illustrated by FIG. 2, there is always a possibility that the exhaust check valve 15 may leak or malfunction to permit reverse flow from the reservoir into the cylinder 10 and that such reverse flow may be suflicient to raise the air pressure in the cylinder '10 sufiiciently to stall the piston. In this regard, a feature of the invention is the concept of the inner circumferential lands 58 and 66 of the two sleeve extensions 18 being dimensioned relative to the periphery of the valve spool to permit a desirable degree of leakage in the event that the piston stalls in its first stage of either of its opposite strokes. The leakage permits hydraulic pressure to build up on the secondary input surface at the end of the corresponding sleeve extension and thus causes initiation of the second stage before the piston advances far enough to uncover the corresponding valve spool ports 56 and 65. It is contemplated that the leakage will be of such magnitude that the piston merely pauses for a few seconds whenever there is any such tendency for the piston to stall.

With reference to the 3-way valve action, the valve spool is maintained in its rightward position during the initial portion of the leftward piston stroke, the radial port 70 at the left end of the valve spool being in communication with the high pressure inlet port 30 to maintain high pressure in the left hydraulic chamber 44. At the same time, the radial port 74 at the right end of the spool is in communication with the low pressure inlet port 38 to maintain low pressure in the right hydraulic chamber 48.

The two hydraulic chambers 44 and 48 which control hydraulic pressure on the opposite ends of the valve spool constitute a first means to bias the valve spool hydraulically to its opposite limit positions in turn. A second means to bias the valve spool hydraulically to its opposite limit positions in turn is provided by differential areas of the valve spool as will now be explained.

In FIG. 1, the diameter A of the central portion of the valve spool is appreciably smaller than the diameters B of the two opposite longitudinal portions of the valve spool. In FIG. 2 where the piston is at an early stage in its leftward stroke the dilference between the cross-sectional areas at the diameters A and B creates a rightward force on the valve spool because at this time the land 58 of the right sleeve extension 18 cuts off hydraulic flow to the region rightward from the land. When the piston advances to the second stage shown in FIG. 3, the hydraulic bias arising from the differential areas disappears because the land 58 of the right sleeve extension uncovers the valve ports 65.

Throughout the first stage and nearly all of the second stage of a piston stroke, one of the hydraulic chambers 44, 48 is pressurized to maintain the valve spool at one of its limit positions. The differential cross-sectional areas of the valve spool are effective to create hydraulic bias in the same direction but only during the first stage.

The forces acting on the valve spool throughout its range of opposite shifts are indicated diagrammatically in FIG. 4 where a central shaded vertically extending area indicates the neutral zone in the travel of the valve spool wherein the valve spool cuts off hydraulic flow to both sides of the piston. What may be termed a rightward balanced zone is indicated in shading at 82 and a similar leftward balanced zone is indicated at 84. Between the neutral zone 80 and the hightward balanced zone 82, a vertical line 85 indicates What may be termed a rightward intermediate position of the valve spool where hydraulic force becomes effective to shift the valve spool leftward away from the piston and the vertical line 86 between the neutral zone 80 and the leftward balanced zone 84 indicates a leftward intermediate position of the valve spool.

With reference to the forces on the valve spool that are involved in the leftward stroke of the piston, the leftward arrow 88 represents the initial leftward movement of the valve spool from its right limit position that is carried out by the piston engaging the valve spool as the piston approaches its left limit position and the rightward arrow 90 represents the rightward hydraulic bias on the spool that opposes the mechanical leftward shift by the piston, this bias being created by hydraulic pressure in the chamber 44 at the left end of the valve spool. The leftward arrow 92 represents the leftward hydraulic force on the spool that is created by pressure in the hydraulic chamber 48 on the right end of the spool. It is to be noted that the two opposite arrows 90 and 92 overlap and thus create the rightward balanced Zone 82.

When the leftward mechanical shift of the valve spool by the piston that is indicated by the arrow 88 extends leftward beyond the rightward neutral zone 82, the leftward hydraulic force represented by the arrow 92 takes over to continue the shift of the valve spool towards its left limit position.

The rightward intermediate position represented by the line 85 is the point in the leftward travel of the valve spool where the hydraulic pressure in the right hydraulic chamber 48 takes over and moves the valve spool out of engagement with the leftwardly moving piston. It is apparent that this point may be anywhere between the right balanced zone 82 and the null zone 80 because it depends on the rate of movement of the piston and the rapidity with whic hthe leftward hydraulic bias is developed. Since the piston is slowed down by the progressive restriction of hydraulic flow to the piston and since pressure changes in the two hydraulic chambers 44 and 48 are rapid, the two intermediate positions 85 and 86' are closely adjacent the corresponding balanced zones '82 and 84.

When the leftward hydraulic end pressure 92 shifts the valve spool leftward past the neutral zone 80, hydraulic flow is directed to the left side of the piston and consequently the leftward hydraulic force represented by the arrow 94 is created by the previously mentioned differential cross-sectional areas of the valve spool.

Both the leftward hydraulic end pressure 92 and the leftward hydraulic differential pressure 94 not only urge the valve spool to its left limit position but also thereafter maintain the valve spool at its left limit position during the rightward stroke of the piston. As heretofore pointed out, the leftward hydraulic differential pressure 94 terminates when the rightwardly moving piston enters its second stage but the leftward hydraulic end pressure 92 endures until the piston approaches its right limit position.

The valve spool forces that are involved in the rightward shift of the spool from its left limit position to its right limit position at the end of its rightward piston stroke are, of course, the reverse of the above discussed forces. Thus the rightward mechanical force of the piston on the valve spool is represented by the rightward arrow 95 and the opposing leftward hydraulic end pressure on the valve spool is represented by the leftward arrow 96. The rightward hydraulic end pressure on the valve spool that is effective after the valve spool passes the leftward balanced zone 84 is indicated by the rightward arrow 98 and the subsequently effective rightward hydraulic differential pressure on the valve spool is represented by the rightward arrow 100.

FIG. shows the positions of the valve spool ports relative to the cooperating ports of the surrounding fixed structure when the leftwardly shifting valve spool enters the rightward balanced zone '82. Up to the point where the leftwardly shifting valve spool reaches the balanced zone 82, the valve spool is biased towards its right limit position by hydraulic pressure in the chamber 44 at the left end of the spool. When the spool reaches the rightward balanced zone 82, the radial port 70 of the spool is still in communication with the high pressure inlet port 30 as shown in FIG. 5. At the same time, the radial port 74 on the right end of the spool is in communication with the high pressure inlet port 32 to pressurize the chamber 48 and thus balance the two end forces on the spool.

It is to be noted that the valve spool land 102 between the radial ports 62. and 74 of the valve spool is centralized relative to the fixed high pressure inlet port 32 and the land is slightly narrower in width than the width of the high pressure inlet port 32 to permit high pressure hydraulic flow not only to the right hydraulic chamber 48- but also into the radial ports 62 for high pressure hydraulic flow to the right side of the leftwardly moving piston. It is apparent then that although the valve spool is balanced with respect to hydraulic end pressure thereon, nevertheless the valve spool cannot stall in this position because continued flow from the high pressure inlet port 32 into the radial port 62 will keep the piston moving to carry the valve spool beyond the rightward balanced zone 82. This fact is also apparent in the diagram in FIG. 4.

FIG. 6 shows the relationships of the valve spool ports to the surrounding fixed ports after the leftward hydraulic force 92 in FIG. 4 has taken over and has shifted the valve spool slightly to the left of the neutral zone 80. In FIG. 6 high pressure flow from inlet port 32 to the rightward hydraulic chamber 48 is increasing as the valve ports 74 shift towards the center of the high pressure inlet port 32. On the other hand, the left valve chamber 44 is at low pressure because both the left valve port 70 and the peripheral port 72 are in communication with the low pressure outlet port 35. At the same time, restricted flow from the high pressure inlet port 30 into the valve port 54 starts building up hydraulic pressure for the ensuing rightward piston stroke.

FIG. 7 which is similar to FIG. 5, shows the relationships of the valve spool ports to the fixed ports when the rightwardly shifted valve spool is in the leftward balanced zone 84 of FIG. 4. FIG. 8, which is similar to FIG. 6, shows the relationships of the valve spool ports to the cooperating fixed ports when the rightwardly shifting valve spool moves slightly past to the neutral zone '80.

For the purpose of explaining the advantages of the two-stage operation of the compressor, FIG. '9 shows how the hydraulic input horsepower varies with the position of the piston if the second stage were omitted, i.e. if the primary input area of the piston were not augmented by the secondary input area. In FIG. 9 it is assumed that the pressure in the air cylinder 10 is raised to the pressure'in the air reservoir of the compressed air system when the piston stroke is 83% completed. It is to be noted that the hydraulic input horsepower that is required to complete the single stage stroke is nearly 5.9. It is further important to note that the work performed by the piston is theshaded area below the curve 104 and that the relatively large area above the curve 104 represents the throttling osses.

In FIG. 10 which is on the same scale as FIG. 9, it is assumed that the second stage begins when the piston stroke is 60% completed and that the maximum air pressure is reached when the piston strokeis 83% completed. It is to be noted that the maximum hydraulic input horsepower is not quite 3.8. Here again the shaded area below the curve 105 represents the work that is accomplished and the relatively small area above the curve represents the throttling losses.

It is readily apparent that the two-stage operation is more efficient because it results in increased work relative to the energy input and not only results in lower throttling losses but also requires lower input power. In addition, it is to be borne in mind that one problem in the operation of a free piston compressor is to provide adequate deceleration of the piston at the end of each opposite stroke and it is inherent in the two-stage operation that the piston velocity drops when the second stage is entered because of the increased input area that is exposed to the high pressure hydraulic fluid.

My description in specific detail of the presently preferred embodiment of the invention will suggest various changes, substitutions and other departures from my disclosure.

1 claim:

1. In a hydraulically actuated compressor, the combination of:

a free piston movable between left and right limit positions, said piston having a first portion reciprocative in an air chamber that is equipped with intake and discharge ports and having a second portion for hydraulic actuation;

a 4 way hydraulic valve including a 4-way valve member to control the piston, said valve member having a first limit position to drive the piston rightward, a second opposite limit position to drive the piston leftward and a central neutral position;

hydraulic means to shift the valve member to its first limit position in response to movement of the valve member from its second limit position to an intermediate position toward its second limit position from its neutral position;

hydraulic means to shift the'valve member to its second limit position in response to movement of the valve member from its first limit position to an intermediate position towards its first limit position from its neutral position; and

means operable by the piston to shift the valve member from its second limit position to its first mentioned intermediate position in response to approach of the piston to Its ieft limit position and to shift the valve from its first limit position to its second mentioned intermediate position in response to approach of the piston to its right limit position.

2. A combination as set forth in claim 1 in which said leftward acting hydraulic means includes means to subject the right end of the valve member to hydraulic pressure and the rightwardly acting hydraulic means includes means to subject the left and of the valve member to hydraulic pressure.

3. A combination as set forth in claim 2 which includes two 3-way valves to control the application of hydraulic pressure to the opposite ends of the 4-way valve member, each of said 3-way valves being a valve member that is integral with the 4-way valve member.

4. A combination as set forth in claim 3 in which the 3-way valve members are opposite end portions of the 4-way valve member.

5. A combination as set forth in claim 1 in which said valve member is shaped and dimensioned relative to surrounding structure to provide differential areas subjected to hydraulic pressure to urge the valve member to its left limit position during initial rightward movement of the piston and to urge the valve member to its right limit position during initial leftward movement of the piston.

'6. A combination as set forth in claim 5 in which the leftwardly acting hydraulic means includes means to subject the right end of the valve member to hydraulic pressure and the rightwardly acting hydraulic means includes means to subject the left end of the valve member to hydraulic pressure.

7. A combination as set forth in claim 1 in which said 4-way valve member reciprocates along the axis of the piston, the piston being of annular configuration and surrounding the valve member,

and in which means fixedly carried by the valve member projects radially into the path of reciprocation of the piston for shift of the valve member by the piston.

8. A combination as set forth in claim 1 which includes means to increase the effective area of said second portion of the piston in response to arrival of the piston at a predetermined intermediate point in each of its opposite strokes.

9. A combination as set forth in claim 8 which includes means providing a leakage path to increase the effective area of said second portion of the piston in the event the piston stalls before it reaches the predetermined intermediate position.

10. In a hydraulically actuated air compressor, the combination of:

a piston having a first portion reciprocative in an air chamber that is equipped with intake and discharge ports and having a second portion for hydraulic actuation;

a valve member that is movable to left and right limit positions from a neutral position to drive the piston rightwardly and leftwardly respectively;

fixed structure surrounding at least a portion of the valve member and having high pressure inlet and loW pressure outlet ports for hydraulic fluid,

said valve member having a first set of fluid passages and spaced peripheral ports cooperative with said inlet and outlet ports for 4-way valve action to direct high pressure hydraulic fluid to the opposite sides in turn of said second portion of the piston for reciprocation of the piston;

leftwardly acting hydraulic means to urge the valve member to its left limit position for driving the piston rightward;

right-wardly acting hydraulic means to urge the valve member to its right limit position for driving the piston leftward,

said valve member having a second set of fluid passages and spaced peripheral ports cooperative with said inlet and outlet ports for 3-Way valve action to control said two hydraulic means for shifting the valve member in opposite directions,

said valve member having a rightward intermediate position rightward from its neutral position at which its second set of passages and ports release hydraulic fluid from the rightwardly acting hydraulic means and supply high pressure hydraulic fluid to the leftwardly acting hydraulic means to cause the valve member to shift to its left limit position,

said valve member having a leftward intermediate position leftward from its neutral position at which its second set of passages and ports release hydraulic fluid from the rightwardly acting hydraulic means and supply high pressure hydraulic fluid .to the rightwardly acting hydraulic means to shift the valve member to its right limit position,

whereby forcibly moving the valve member from its right limit position to its rightward intermediate position in opposition to the rightwardly acting hydraulic means results in shift of the Valve member to its left limit position by the leftwardly acting hydraulic means to reverse the movement of the piston from left to right and forcibly moving the valve member from its left limit position to its leftward intermediate position in opposition to the leftwardly acting hydraulic means results in shift of the valve member to its right limit position by the rightwardly acting hydraulic means to reverse the movement of the piston from right to left; and means to shift the valve member from its right limit position to its rightward intermediate position in re sponse to movement of the piston to its left limit position and to shift the valve member from its left limit position to its leftward intermediate position in response to movement of the piston to its right limit position, thereby to cause repeated reciprocation of the piston. 11. A combination as set forth in claim 10 in which said first set of passages and ports of the valve member are positioned on the valve member to cooperate with 1 1 said inlet ports of the fixed structure to progressively restrict the .fiow of high pressure hydraulic fluid to said second portion of the piston as the piston approaches each of its two opposite limit positions.

12. A combination as set forth in claim which includes means to operatively connect the piston to the valve member to shift the valve member from its right limit position to its rightward intermediate position as the piston approaches its left limit position and to operatively connect the piston to the valve member to shift the valve member from its left limit position to its leftward inter mediate position as the piston approaches its right limit position.

13. A combnation as set forth in claim 12 in which means fixedly carried by the valve member is positioned in the path of reciprocation of means fixedly carried by the piston to cause the piston to push the valve means from its right limit position to its rightward intermediate position and to push the valve means from its left limit position to its leftward intermediate position.

14. A combination as set forth in claim 13 in wheich said valve member reciprocates along the axis of the piston, the piston being of annular configuration 'and surrounding the valve member,

and in which the means fixedly carried by the valve member projects radially into the path of reciprocation of the piston.

15. A combination as set forth in claim 14 in which said valve member is dimensioned relative to said fixed structure to provide differential cross-sectional areas for subject to the high pressure hydraulic fluid to bias the valve member to its right limit position as the piston initiates its leftward stroke and to bias the valve member to its left limit position as thepiston initiates its rightward stroke.

16. A combination as set forth in claim 14 in which the piston includes two opposite axial sleeve extensions surrounding the valve member, said sleeve extensions forming longitudinally spaced shoulders facing away from each other;

and in which the valve member has longitudinally spaced peripheral shoulders facing towards each other to cooperate with the shoulders of the sleeve extensions for shifting the valve member.

17. A combination as set forth in claim 16 in which said sleeve extensions respectivelyform auxiliary surfaces for hydraulic actuation of the piston structure;

18. A combination as set forth in claim 17 in which .each of said sleeve extensions is dimensioned relative to the valve member to permit leakage flow from the corresponding auxiliary port of the valve member to the corresponding auxiliary surface of the sleeve extension while the sleeve extension is covering the auxiliary ports,

whereby any tendency for the piston to be stalled by opposed air pressure before the piston reaches said intermediate point results in delayed increase in the hydraulic thrust to move the piston to the intermediate point. 19. A combination as set forth in claim 10 in which said fixed structure encloses each end of the valve member and provides a right end wall confronting the right end of the valve member and a left end wall confronting the left end of the valve member;

and in which the leftwardly acting hydraulic means thrusts against the right end wall and the leftwardly acatilng hydraulic means thrusts against the left end w t 12 20. A combination as set forth in claim 19 in which theleftwardly acting hydraulic means includes a plunger slidingly mounted in the right end of the valve member to thrust against said right end wall and in which the rightwardly acting hydraulic means includes a plunger slidingly mounted in the right end of the valve member to thrust against said right end wall and in which the rightwardly acting hydraulic means includes a plunger slidingly mounted in the left end ofthe valve member to thrust against said left end wall,

said second set of passages and ports of the valve mem ber cooperating with said inlet and outlet ports of the fixed structure to apply high pressure'to the.inner ends of the plungers. 21. A combination as set forth in claim 10 in which said high pressure inlet ports of the fixed structure comprise two high pressure inlet ports near the opposite ends respectively of the range of movement of the valve member; in which said low pressure outlet ports comprise two pairs of low pressure outlet ports near the opposite ends respectively of the range of movement of the valve member, each of the high pressure inlet ports being between and adjacent the two corresponding low pressure outlet ports; in which the first set of passages and ports in the valve member cooperates with each of the two high pres sure inlet ports and one of the low pressure outlet ports adjacent thereto; and in which the second set of passages and ports in the valve member cooperates with each of the two high pressure inlet ports and with the other of the two low pressure outlet ports adjacent thereto.

22. A combination as set forth in claim 21 in which the valve member has two lands near its opposite ends respectively, each of the lands separating two ports of said two sets' respectively;

and in which the width of each of said lands is slightly less than the width of said high pressure inlet ports to permit positioning of the valve member for supplying high pressure hydraulic fluid to the two sets simultaneously.

23. In an air compressor wherein an output aera of a piston acts against air pressure, the improvement comprising:

the piston having a primary input area and a second ary input area to receive actuating hydraulic pressure;

means to place the primary input area of the piston in communication with high pressure hydraulic fluid to provide an initial ratio of input area to output area to carry out an initial portion of the piston stroke for a first stage of air compression; and

means to place the secondary input area in addition to the primary input area in communication with high pressur hydraulic fluid at a predetermined intermediate point in the piston stroke to raise the ratio of the input area to the output area to carry out a second stage of air compression for the remainder of the piston stroke.

24. An improvement as set forth in claim 23 which includes provision for a leakage path for flow. of high pressure hydraulic fluid to the secondary input area while said valve means is closed whereby if the initial ratio of input area to output area is insuflicient to overcome the resisting air pressure, leakage to the secondary input area initiates the second stage of air compression at a point in the pston stroke earlier than said predetermined intermediate point.

25. In a hydraulically actuated combination of:

a free piston of annular configuration having opposite output areas in an outer radial zone and opposite primary input areas in an inner radial zone, said piston having opposite axial extensions in an intermediate air compressor, the

radial zone providing opposite secondary input areas; and 4-way valve means including a valve spool reciprocative from opposite limit positions along the axis of the piston inside said axial extensions to control the application of hydraulic pressure to said primary input areas of the piston to stroke the piston in opposite directions,

said valve spool being cooperative with said axial extensions to apply high hydraulic pressure to said secondary input areas at intermediate points of the piston strokes.

26. A combination as set forth in claim 25 which includes cooperative means on the axial extensions and the valve spool respectively to shift the valve spool from each of its limit positions part way to its opposite limit position as the piston completes its opposite strokes.

27. A combination as set forth in claim 26 which includes hydraulic means to complete the shift of the valve spool from each of its limit positions to its opposite limit position in response to shift of the spool by one of said axial extensions.

28. A combination as set forth in claim 27 which includes means cooperative with the valve spool for 3-Way valve action to operate said hydraulic means.

References Cited UNITED STATES PATENTS 15 ROBERT M. WALKER, Primary Examiner US. Cl. X.R. 

